It has been known for many years that heat treatment of wood will improve the wood's characteristics, specifically with respect to durability.
During the last 30 years intensive research into this has improved and refined the knowledge and the methods, and especially Finland has been a leading force in this development. The Finnish Thermo Wood Association in 2003 issued a book with the title Thermo Wood® Handbook. This handbook has since become the standard reference when it comes to heat treatment of wood and furthermore the Thermo Wood® method is by far the most widespread method of heat treating wood.
The thermo treatment method does in general terms include three phases:
A first phase where the wood to be treated is placed in a treatment chamber wherein the temperatures increase, first to around 100° C. and thereafter the temperature is slowly raised to 130° C. where high temperature drying takes place and the moisture content in the wood decreases to nearly 0. This first phase lasts approximately 36 hours.
Following the first phase is the second phase where the temperature is further increased to a temperature between 185° C. and 215° C. The elevated temperature remains constant for some hours depending on the desired outcome.
Altogether the second phase lasts approximately 16-17 hours.
Finally, the third phase is the cooling and moisture conditioning phase where once the temperature has fallen to below 80 or 90° C. re-moisturizing of the wood takes place up to a moisture content in the wood of approx. 4-7%.
After this 20+ hour treatment in the third phase the treated wood has obtained some advantageous characteristics which the wood did not have before the treatment. Most importantly the durability of the wood is increased, but also insulation properties and resistance to decay and tear and wear due to environmental influences are dramatically reduced.
Thermo-wood manufactured according to the prior art methods, even though a longer effective service life is obtained, does still have shortcomings when compared to traditional impregnated wood. In addition to the thermo treatment it is also necessary to surface treat the wood in order to protect the wood from the environment.